1. Field of the Invention
This invention relates to an electron gun for a color picture tube, and more particularly a focus electrode structure of an electron gun, which is capable of preventing deterioration of resolution of a picture tube due to changes in electrons beam paths resulting from thermal deformation of electrodes during operation of the electron gun.
2. Description of the Prior Art
Generally, a color picture tube is made of glass and comprises a panel 11 having a phosphor screen 12 on applied to its inner surface, and a funnel 13 having a neck portion of a reduced cross-section, as shown in FIG. 1 of the accompanying drawings. In addition, the tube includes an electron gun 8 mounted within the neck portion of the funnel to emit thermions and a color selection shadow mask 14 disposed in predetermined spaced relation to the phosphor screen 12. Moreover, a deflection yoke 15 for deflecting an electron beam is mounted around the rear portion of the funnel 13.
The detailed construction of the electron gun 8 used in the color picture tube is shown in FIG. 2. The gun comprises three coplanar cathodes 9 each associated with a heater 10, a control electrode 4 and a screen electrode 5 disposed in front of the cathodes 9, and a focus electrode 3, an anode 6 and a shield cup 7 arranged in front of the screen electrode 5, in the order recited. The focus electrode 3 includes an electrode body 1 and a cap 2. The electrodes 3,4,5 and 6 are arranged in a line along electrical insulator rods(not shown), such as bead glasses, and fixedly mounted on the rods in a predetermined space relation to one another. Further, each of the electrodes has three electron beam passage apertures disposed in line in a common horizontal plane.
With the color picture tube thus constructed, as the heaters 10 are energized to generate heat, the cathodes 9 emit thermions which pass through the respective electrodes of the electron gun 8 as three coplanar electron beams. Then the beams pass through the shadow mask 14 to the screen 12 with phosphor coated thereon to cause luminescence of the phosphor material, thereby producing a picture image.
Moreover, there are formed within the electron gun 8 two electrostatic focusing lens parts, one of them being a pre-focusing lens part formed by a potential difference between applied voltage of the screen electrode 5 and voltage of the focus electrode 3, and the other being a main focusing lens part formed by a potential difference between voltage of the focus electrode 3 and voltage of the anode 6. The pre-focusing lens part acts to reduce diffusion of the electron beams emitted from the cathodes 9, and the main focusing lens part acts to focus the beams on the screen. Therefore, if central axes of the paths of the electron beams do not coincide exactly with a central axis of the main focusing lens, haze is formed at a beam spot, resulting in considerable deterioration of resolution of the color picutre tube.
Referring to FIG. 5, illustrating in plan view the electrode body of the focus electrode used in the prior art electron gun, the electrode body 1 has three circular apertures a, b, and c arranged in line at equal distances t.sub.1 to permit passage of three electron beams. Each of the apertures is approximately 1.5 mm in diameter. The prior art focus electrode having the beam passage apertures of the same circular shapes as described above have exhibited the following drawback. That is, during operation of the pricture tube, high temperature(about 780.degree..about.820.degree. C.) generated by the heaters 10 gives rise to thermal deformation of the control and screen electrodes 4 and 5. As a result, the control and screen electrodes 4 and 5 suffer thermal expansion in the direction indicated by the arrows in FIG. 4, as a result of which the central axes of the side apertures b and c of the electrodes are displaced to the positions shown by the dotted lines, whereby the electron beams are directed with their central axes deviated from the central axes of the main lenses M, thereby producing haze at the beam spot on the screen and lower resolution of the picture tube. As a result, while the center apertures a of the control and screen electrodes undergo only enlargement in diameter under the influence of the heat without changes of their centers, the side apertures b and c are subjected to displacement of their centers. As a result, the electrons emitted from the cathodes 9 pass through the side apertures along the paths offset from their normal paths, and then deviate from the central axes of the main lenses to be deflected from the normal paths toward the center of the electrode body.